Question

a convex lens from a real image 4tlme magnified at a distance of 60cm from the lens .calculate the focal length and the power of the lens​

Answer 1

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Answer 2

Question

A convex lens from a real image 4 times magnified at a distance of 60cm from the lens. Calculate the focal length and the power of the lens.

Solution-

Given that, magnification of convex lens is 4. Also said that image formed is magnified and real. So, magnification of convex lens is -4.

Also given that, image distance from the lens is 60cm (v = 60).

We have to find the focal length (f) and power of the lens (P).

For lens magnification:

m = v/u

Substitute the known in the above formula,

→ -4 = 60/u

→ u = 60/(-4)

→ u = -15

Therefore, the object distance from the lens is -15 cm.

Using lens formula:

1/f = 1/v - 1/u

Substitute u = -15 and v = 60 in above formula to find the value of focal length.

→ 1/f = 1/60 - 1/(-15)

→ 1/f = 1/60 + 1/15

→ 1/f = (1 + 4)/60

→ 1/f = 5/60

→ 1/f = 1/12

→ f = 12

Therefore, the focal length of convex lens is 12 cm.

Power of lens is defined as the reciprocal of focal length.

P = 1/f

From above calculations we have focal length = 12 cm So,

→ P = 1/12

→ P = 0.083 (in m)

→ P = 0.083 × 100 (in cm)

→ P = 8.3

Therefore, the power of lens of convex lens is 8.3D.